1. Field of the Invention
The present invention relates to a processing method for cleaning an object to be processed after it has been subjected to a film-formation process.
2. Description of Related Art
Aluminum wiring is mainly used as the wiring pattern of an integrated circuit and an SiO2 or SiOF film is used as an inter-layer insulating film for insulating that wiring. These films are formed by using a plasma apparatus that performs an electron cyclotron resonance (ECR) processing, as shown in FIG. 6 by way of example.
In FIG. 6, microwaves M at, for example, 2.45 GHz are supplied into the interior of a plasma generation chamber 1A of a vacuum vessel 10, and also a magnetic field of, for example, 875 Gauss is applied thereto by an electromagnetic coil 12, so that the mutual interaction (resonance) between the microwaves and the magnetic field causes a plasma to be formed from a plasma gas such as Ar or O2 and also a film-formation gas such as SiH4 which is introduced into a film-formation chamber 1B. This forms a film on a semiconductor wafer (hereinafter referred to simply as xe2x80x9cwaferxe2x80x9d) W that is mounted on a mounting stand 13 (see FIG. 7A). This mounting stand 13 is provided with a foil electrode 15 and a resistance heater 16 for heating the wafer W to a predetermined temperature, and it is covered by an insulating layer 14 formed of a material such as a polyimide, alumina, or aluminum nitride, and is configured so that the surface thereof acts as an electrostatic chuck.
If the above film-formation step is repeated, a film S will adhere to the periphery of the mounting stand 13 and the inner walls within the film-formation chamber 1B, as shown in FIG. 6, and this film S will crack and generate particles when it reaches a certain thickness. A cleaning process to remove the adhered film S is therefore performed every time a dozen or so wafers W have been processed. The cleaning process for removing an SiO2 film or the like, is done by sending a fluorine-based gas such as CF4 or NF3 as a cleaning gas into the film-formation chamber 1B, activating the cleaning gas by a plasma, then causing radicals to react with the adhered film to remove it (see FIG. 7C).
Unfortunately, if the mounting stand 13 is exposed to a plasma, the surface of the electrostatic chuck will be struck by the plasma and will be roughened thereby. This will change the attractive force with respect to the wafer W and the heat conduction within the surface, thus worsening the surface uniformity with which the film thickness is formed on the wafer W. In order to protect the surface of the mounting stand 13 from the plasma, it has been considered to place a dedicated protective wafer CW on the mounting stand 13 as shown in FIG. 7C, and perform the cleaning. An alumina (Al2O3) disk 17 having an aluminum (Al) layer 17a formed on the lower surface thereof is used as this protective wafer CW, and this protective wafer CW is held by the electrostatic attraction of the aluminum layer 17a onto the mounting stand 13.
The alumina that is the material of the protective wafer CW is a ceramic. So it is weak with respect to thermal shocks but the protective wafer CW is attracted onto the mounting stand 13. Therefore, even though the surface temperature is raised abruptly by the plasma, heat escapes towards the mounting stand and thus the thermal shocks can be reduced to a certain extent.
However, there is a tendency towards increasing the processing temperatures during this film-formation process, in order to improve the quality of the film formed thereby, so that the wafer W is heated to approximately 350xc2x0 C. by the resistance heater 16 and the plasma during the process of forming an SiOF film. The temperature of the surface of the mounting stand 13 at this point is approximately 200xc2x0 C. In this case, the electrostatic attraction ensures that the thermal shock on the alumina can be reduced to a certain extent, but, even so, the surface temperature of the mounting stand 13 has an upper limit of about 150xc2x0 C. and thus any temperature above that will cause the alumina to crack from thermal shock. Therefore, to prevent cracking of the protective wafer CW, the cleaning must be performed after the surface temperature of the mounting stand 13 has been reduced to about 150xc2x0 C., but this means that it takes time before the cleaning can start.
When the cleaning has ended, fluorine from within the cleaning gas is still dispersed within the vacuum vessel in a radical state, or it has adhered to the inner walls thereof. Thus, if the film-formation processing is performed immediately after this cleaning, the fluorine will be incorporated into the processing of the first few wafers W, particularly the very first wafer, which will impair the film quality. In such a case, pre-coating is performed after the cleaning in order to form a pre-coating film on the inner walls of the vacuum vessel 10 and thus fix the fluorine to the inner wall surfaces, as described below. This pre-coating is a film-formation processing that acts as a preliminary step. If the main film-formation processing step is to create an SiO2 film or the like, the pre-coating film is formed of SiO2 or SiOF or the like.
To prevent the pre-coating film from forming on the surface of the mounting stand 13, a protective wafer PW for pre-coating, made of a material such as a silicon wafer, is placed on the surface of the mounting stand 13 (see FIG. 7D). After the pre-coating has ended, a wafer W for processing is mounted on the mounting stand 13 and once again the formation of an SiO2 film or the like is performed (see FIG. 7E).
The pre-coating gas used for this pre-coat is the same as the film-formation gas. So an alumina wafer can be used as the protective wafer PW. Therefore an alumina protective wafer PW is used for the pre-coating. The processing is performed while the temperature of the mounting stand 13 is being raised until it is close to the temperature of the film-formation process.
However, since the pre-coating takes only a short time, the temperature of the mounting stand 13 cannot rise to the temperature required for the film-formation during that time, and thus more time elapses before the film-formation processing can start. If an alumina protective wafer PW is used in this manner, it takes time to adjust the temperature before each of the cleaning processing and the film-formation processing can start, which reduces the throughput.
An object of the present invention is to provide a processing method that enables cleaning while protecting the mounting surface of the mounting stand for the object to be processed, such as the surface of the electrostatic chuck. In addition to the above object, another object of the present invention is to provide a processing method that enables processing at a high throughput.
The present invention therefore provides a processing method for an object to be processed, comprising the steps of: mounting an object to be processed on a mounting stand provided within a film-formation chamber; performing a film-formation process on the object to be processed by supplying a film-formation gas into the film-formation chamber; mounting a protective plate of aluminum nitride of the same size as the object to be processed on the mounting stand; and then introducing a cleaning gas into the film-formation chamber to form a plasma, thereby cleaning the interior of the film-formation chamber.
After this cleaning process, it is possible to perform a further step of introducing a film-formation gas into the film-formation chamber to perform a pre-coating process, while the protective plate is still mounted on the mounting stand.